The following description of the background is provided to aid in understanding, but is not admitted to be, or to describe, prior art. All publications and their cited references are incorporated by reference in their entirety.
Thyroid hormones (TH) are synthesized in the thyroid in response to thyroid stimulating hormone (TSH), which is secreted by the pituitary gland in response to various stimulants (e.g., thyrotropin-releasing hormone (TRH) from the hypothalamus). Thyroid hormones are iodinated O-aryl tyrosine analogues excreted into the circulation primarily as 3,3′,5,5′-tetraiodothyronine (T4). T4 is rapidly deiodinated in local tissues by thyroxine 5′-deiodinase to 3,3′,5′-triiodothyronine (T3), which is the most potent TH. T3 is metabolized to inactive metabolites via a variety of pathways, including pathways involving deiodination, glucuronidation, sulfation, deamination, and decarboxylation. Most of the circulating T4 and T3 is eliminated through the liver.
THs have profound physiological effects in animals and humans. Hyperthyroidism is associated with increased body temperature, general nervousness, weight loss despite increased appetite, muscle weakness and fatigue, increased bone resorption and enhanced calcification, and a variety of cardiovascular changes, including increased heart rate, increased stroke volume, increased cardiac index, cardiac hypertrophy, decreased peripheral vascular resistance, and increased pulse pressure. Hypothyroidism is generally associated with the opposite effects.
The biological activity of THs is mediated largely through thyroid hormone receptors (TRs). TRs belong to the nuclear receptor superfamily, which, along with its common partner, the retinoid X receptor, form heterodimers that act as ligand-inducible transcription factors. Like other nuclear receptors, TRs have a ligand binding domain and a DNA binding domain and regulate gene expression through ligand-dependent interactions with DNA response elements (thyroid response elements, TREs). Currently, the literature shows that TRs are encoded by two distinct genes (TRα and TRβ), which produce several isoforms through alternative splicing (Williams, Mol. Cell Biol. 20(22):8329-42 (2000); Nagaya et al., Biochem. Biophys. Res. Commun. 226(2):426-30 (1996)). The major isoforms that have so far been identified are TRα-1, TRα-2, TRβ-1 and TRβ-2. TRα-1 is ubiquitously expressed in the rat with highest expression in skeletal muscle and brown fat. TRβ-1 is also ubiquitously expressed with highest expression in the liver, brain and kidney. TRβ-2 is expressed in the anterior pituitary gland and specific regions of the hypothalamus as well as the developing brain and inner ear. In the rat and mouse liver, TRβ-1 is the predominant isoform (80%). The TR isoforms found in human and rat are highly homologous with respect to their amino acid sequences which suggest that each serves a specialized function.
TSH is an anterior pituitary hormone that regulates thyroid hormone production. TSH formation and secretion is in turn regulated by the hypothalamic TRH. TSH controls the uptake of iodide by the thyroid, the subsequent release of iodinated thyronines from thyroglobulin (e.g., T3, T4) as well as possibly the intrapituitary conversion of circulating T4 to T3. Compounds that mimic T3 and T4 can negatively regulate both TSH and TRH secretion resulting in suppression of TSH levels and decreased levels of T3 and other iodinated thyronines. Negative regulation of TSH is postulated based on co-transfection and knockout studies (Abel et al., J. Clin. Invest. 104:291-300 (1999)) to arise through activation of the thyroid receptor TRY, possibly the isoform TRβ-2, which is highly expressed in the pituitary.
The most widely recognized effects of THs are an increase in metabolic rate, oxygen consumption and heat production. T3 treatment increases oxygen consumption in isolated perfused liver and isolated hepatocytes. (Oh et al., J. Nutr. 125(1):112-24 (1995); Oh et al., Proc. Soc. Exp. Biol. Med. 207(3): 260-7 (1994)) Liver mitochondria from hyperthyroid rats exhibit increased oxygen consumption (Carreras et al., Am. J. Physiol. Heart Circ. Physiol. 281(6):H2282-8 (2001) and higher activities of enzymes in the oxidative pathways (Dummler et al., Biochem. J. 317(3):913-8 (1996), Schmehl et al., FEBS Lett. 375(3):206-10 (1995), Harper et al., Can. J. Physiol. Pharmacol. 72(8):899-908 (1994)). Conversely, mitochondria from hypothyroid rats show decreased oxygen consumption. Increased metabolic rates are associated with increased mitochondrial biogenesis and the associated 2- to 8-fold increase in mitochondrial mRNA levels. Some of the energy produced from the increased metabolic rate is captured as ATP (adenosine 5′-triphosphate), which is stored or used to drive biosynthetic pathways (e.g., gluconeogenesis, lipogenesis, lipoprotein synthesis). Much of the energy, however, is lost in the form of heat (thermogenesis), which is associated with an increase in mitochondrial proton leak possibly arising from TH-mediated effects on mitochondrial membrane, uncoupling proteins, enzymes involved in the inefficient sn-glycerol 3-phosphate shuttle such as mitochondrial sn-glycerol 3-phosphate dehydrogenase (mGPDH), and/or enzymes associated with proton leakage such as the adenine nucleotide transporter (ANT), Na+/K+-ATPase, Ca2+-ATPase and ATP synthase.
THs also stimulate metabolism of cholesterol to bile acids. Hyperthyroidism leads to decreased plasma cholesterol levels, which is likely due to increased hepatic LDL receptor expression. Hypothyroidism is a well-established cause of hypercholesterolemia and elevated serum LDL. L-T3 is known to lower plasma cholesterol levels. The effects of T3 are attributed to TRβ since TRβ-deficient mice are resistant to T3-induced reduction in cholesterol levels. The effects on cholesterol levels have been postulated to result from direct effects on LDL receptor expression, enzymes involved in conversion of cholesterol to bile acids such as the rate-limiting enzyme cholesterol 7α-hydroxylase (CYP7A) and/or possibly enzymes involved in cholesterol synthesis such as HMG CoA reductase. In addition, THs are known to affect levels of other lipoproteins linked to atherosclerosis. THs stimulate apo AI and the secretion of apo AI in HDL while reducing apo B100. Accordingly, one would expect T3 and T3 mimetics to inhibit the atherosclerotic process in the cholesterol fed animal.
THs simultaneously increase de novo fatty acid synthesis and oxidation through effects on enzymes such as ACC, FAS, and spot-14. THs increase circulating free fatty acids (FFA) levels in part by increasing production of FFAs from adipose tissue via TH-induced lipolysis. In addition, THs increase mitochondrial enzyme levels involved in FFA oxidation, e.g., carnitine palmitoyltransferase 1 (CPT-1) and enzymes involved in energy storage and consumption.
The liver represents a major target organ of THs. Microarray analysis of hepatic gene expression from livers of hypothyroid mice and mice treated with T3 showed changes in mRNA levels for 55 genes (14 positively regulated and 41 negatively regulated) (Feng et al., Mol. Endocrinol. 14(7): 947-55 (2000). Others have estimated that approximately 8% of the hepatic genes are regulated by T3. Many of these genes are important to both fatty acid and cholesterol synthesis and metabolism. T3 is also known to have other effects in liver, including effects on carbohydrates through increased glycogenolysis and gluconeogenesis and decreased insulin action.
The heart is also a major target organ of THs. THs lower systemic vascular resistance, increase blood volume and produce inotropic and chronotropic effects. Overall TH results in increased cardiac output, which may suggest that T3 or T3 mimetics might be of use to treat patients with compromised cardiac function (e.g., patients undergoing coronary artery bypass grafting (CABG) or cardiac arrest) (U.S. Pat. No. 5,158,978). The changes in cardiac function are a result of changes in cardiac gene expression. Increased protein synthesis and increased cardiac organ weight are readily observed in T3-treated animals and represent the side effect of T3 that limits therapeutic use. TRβ knockout mice exhibit high TSH and T4 levels and increased heart rate suggesting that they retain cardiac sensitivity and therefore that the cardiac effects are via TRα. TRα knockouts exhibit reduced heart rates.
THs also play a role in the development and function of brown and white adipose tissue. Both TRα and TRβ are expressed in brown adipose tissue (BAT). THs induce differentiation of white adipose tissue (WAT) as well as a variety of lipogenic genes, including ACC, FAS, glucose-6-phosphate dehydrogenase and spot-14. Overall THs play an important role in regulating basal oxygen consumption, fat stores, lipogenesis and lipolysis (Oppenheimer et al., J. Clin. Invest. 87(1):125-32 (1991)).
TH has been used as an antiobesity drug for over 50 years. In the 1940s TH was used alone, whereas in the 1950s it was used in combination with diuretics and in the 1960s in combination with amphetamines. Hyperthyroidism is associated with increased food intake but is also associated with an overall increase in the basal metabolic rate (BMR). Hyperthyroidism is also associated with decreased body weight (ca. 15%) whereas hypothyroidism is associated with a 25-30% increase in body weight. Treating hypothyroidism patients with T3 leads to a decrease in body weight for most patients but not all (17% of the patients maintain weight).
The effectiveness of TH treatment is complicated by the need for supraphysiological doses of T3 and the associated side effects, which include cardiac problems, muscle weakness and erosion of body mass. Long-term therapy has also been associated with bone loss. With these side effects, the medical community has tended to use thyroxine at low doses as an adjunct to dietary treatments. At these doses, TH has little effect on body weight or BMR.
The effectiveness of T3 to induce weight loss may be attenuated by defects in TH action. In comparison to normal animals, higher T3 doses were required in ob/ob mice to affect oxygen consumption, which was only observed in muscle, with no changes in liver and BAT. (Oh et al., J. Nutr. 125(1):112-24 (1995); Oh et al., Proc. Soc. Exp. Biol. Med. 207(3):260-7 (1994)). These effects were at least partially attributed to decreased uptake of T3 by the liver.
T3 analogues have been reported. Many were designed for use as cholesterol-lowering agents. Analogues that lower cholesterol and various lipoproteins (e.g., LDL cholesterol and Lp(a)) without generating adverse cardiac effects have been reported (e.g., Underwood et al., Nature 324:425-9 (1986)). In some cases the improved therapeutic profile is attributed to increased specificity for the TR-β wherein other cases it may be due to enhanced liver distribution. (Stanton et al., Bioorg. Med. Chem. Lett. 10(15):1661-3 (2000); Dow et al., Bioorg. Med. Chem. Lett. 13(3):379-82 (2003)).
T3 and T3 mimetics are thought to inhibit atherosclerosis by modulating the levels of certain lipoproteins known to be independent risk factors or potential risk factors of atherosclerosis, including low density lipoprotein (LDL)-cholesterol, high density lipoprotein (HDL)-cholesterol, apoAI, which is a major apoprotein constituent of high density lipoprotein (HDL) particles and lipoprotein (a) or Lp(a).
Lp(a) is an important risk factor, elevated in many patients with premature atherosclerosis. Lp(a) is considered highly atherogenic (de Bruin et al., J. Clin. Endocrinol. Metab. 76:121-126 (1993)). In man, Lp(a) is a hepatic acute phase protein that promotes the binding of LDL to cell surfaces independent of LDL receptors. Accordingly, Lp(a) is thought to provide supplementary cholesterol to certain cells, e.g., cells involved in inflammation or repair. Lp(a) is an independent risk factor for premature atherosclerosis. Lp(a) is synthesized in the liver.
Apolipoprotein AI or apoAI is the major component of HDL, which is an independent risk factor of atherosclerosis. apoAI is thought to promote the efflux of cholesterol from peripheral tissues and higher levels of HDL (or apoAI) result in decreased risk of atherosclerosis.
Hyperthyroidism worsens glycemic control in type 2 diabetics. TH therapy is reported to stimulate hepatic gluconeogenesis. Enzymes specific to gluconeogenesis and important for controlling the pathway and its physiological role of producing glucose are known to be influenced by TH therapy. Phosphoenolpyruvate carboxykinase (PEPCK) is upregulated by TH (Park et al, J. Biol. Chem. 274:211 (1999)) whereas others have found that glucose 6-phosphatase is upregulated (Feng et al., Mol. Endocrinol. 14:947 (2000)). TH therapy is also associated with reduced glycogen levels.
TH therapy results in improved non insulin stimulated and insulin stimulated glucose utilization and decreased insulin resistance in the muscle of ob/ob mice. (Oh et al., J. Nutr. 125:125 (1995)).
There is still a need for novel thyromimetics that can be used to modulate cholesterol levels, to treat obesity, and other metabolic disorders especially with reduced undesirable effects.